1. Field of the Invention
The invention relates to a process for producing an .epsilon.-caprolactone-modified hydroxyalkyl acrylate or methacrylate and then a copolymer suitable for coating from the same and another monomer.
2. Description of Prior Arts
Recently, the importance of acrylic paints in the coating field has increased.
A reason therefor is that, as compared with alkyd resins, polyester resins and epoxy resins, the acrylic paints have superior weather resistance, physical properties, chemical resistance and stain resistance. The acrylic coatings are, therefore, used for the production of motorcars, household electric appliances, metals and construction materials in various fields.
Among acrylic resins, an acrylic polyol obtained by polymerizing hydroxyl group-containing monomer is further incorporated with a cross-linking agent reactive with the hydroxyl group, such as polyisocyanate or melamne resin, and used as a cold-setting or baking-setting paint. The hydroxyl group-containing acrylic monomers are indispensable also for obtaining coating films having a high adhesion and gasoline resistance.
As the hydroxyl group-containing monomer, there have been used, for example, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate and hydroxypropyl methacrylate.
However, the acrylic polyols obtained bythe copolymerization of the above-mentioned monomers have an insufficient reactivity with a cross-linking agent, since the hydroxyl group is located at a position very close to the main chain of the stiff acrylic resin skeleton.
Recently, from the viewpoint of saving energy or resources, acrylic paints having higher solidifying properties and lower-temperature setting properties are demanded in the coating field. For satisfying this requirement, it is necessary to increase the reactivity of the hydroxyl group in the acrylic polyols.
For this purpose, 4-hydroxybutyl acrylate of the following formula has been proposed: ##STR1##
However, 4-hydroxybutyl acrylate is expensive and its reactivity is still insufficient. Further, there has been proposed also a lactone-modified acrylic polyol of the following structure obtained by adding .epsilon.-caprolactone to the hydroxyl group of an acrylic polyol resin: ##STR2##
This modified compound has attracted attention as a low-temperature setting, high solid-type paint, since the hydroxyl group is located at a position distant from the main chain. However, the process for reacting the acrylic polyol resin with .epsilon.-caprolactone has many disadvantages. One of them is that the reaction is limited by the type of solvent used, since the acrylic polyol resin is in the form of a solution in a large quantity of the solvent. For example, the addition reaction with .epsilon.-caprolactone requires as high a temperature as possible. However, if a solution of acrylic polyol in toluene solvent is used, the reaction temperature cannot be elevated beyond 110.degree.-120.degree. C. In addition, the reaction velocity is very low, since it is diluted with the solvent. If the quantity of a catalyst for the ring-opening reaction of .epsilon.-caprolactone is increased so as to enhance the reaction velocity, the resin solution is seriously colored and no practically valuable products can be obtained. Further, such a large amount of the catalyst contained in the acrylic polyol paint exhibits quite bad influences on changes in storage stability with time, reactivity with a hardening agent and physical prcperties of the coating film after a long period of time.
In general, the hydroxyl group-containing monomers used for the production of acrylic polyols include hydroxyethyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl methacrylate. A polymer copolymerized with these monomers does not react completely with a cross linking agent because the hydroxyl groups which take part in crosslinking are positioned near the rigid main chain of the acrylic resin skeleton. The insufficient reactivity of the hydroxyl groups results in unsatisfactory coating properties even when the acrylic polyol resin is reduced in molecular weight to increase the solid content. On the other hand, a soft acrylic polyol was synthesized to obtain an elastic coating by incorporating acrylic resin with a large quantity of monomers which lowers the glass transition temperature (Tg). Such an acrylic polyol, however, has a drawback that the resulting coating film is liahle to suffer blocking due to the soft side chains and is easy to become stiff at low temperatures, losing resilience.
In order to overcome these disadvantages, an attempt was made to graft-polymerize alkyd resin to acrylic resin. This attempt was not successful because alkyd resin contains secondary hydroxyl groups which are poor in reactivity and because the resulting coating film has greatly decreased performance in weather resistance and water resistance.
It has also been proposed to copolymerize 4-hydroxybutyl acrylate, as the hydroxyl group-containing monomer, so that the primary hydroxyl groups are positioned far away from the main chain. This proposal is not acceptable because the monomer is expensive and is not sufficient in reactivity.
Recently, there has been proposed a lactone-modified acrylic polyol prepared by the ring opening polymerization of .epsilon.-caprolactone to the hydroxyl group of acrylic polyol resin. This resin is greatly reactive with a crosslinking agent because of the highly reactive primary hydroxyl groups which are positioned far away from the main chain. This resin used to be produced by adding .epsilon.-caprolactone to an acrylic polyol resin and then carrying out ring opening polymerigation in the presence of a catalyst (Refer to Japanese Laidopen Patent No. 48-66194 (1973). ) This resin is also produced by copolymerizing acrylic polyol in .epsilon.-caprolactone and then adding by ring opening .epsilon.-caprolactone to hydroxyl groups of the acrylic polyol in the presence of a catalyst. (Refer to Japanese Laid-open Patent No. 54-133590 (1979). ) Further, there has been proposed a process for carrying out simultaneously ring opening polymerization of .epsilon.-caprolactone and radical polymerization of vinyl monomers by adding a radical initiator , a catalyst for ring opening polymerization of .epsilon.-caprolactone, .epsilon.-caprolactone, hydroxyl group-containing vinyl monomers, other vinyl monomers, and solvent. (Refer to U.S. Pat. No. 4,082,816.) This process, however, has a drawback that the reaction temperature is limited by the boiling point of the solvent used for the polymerization of acrylic monomers. In the case of commonly used solvents such as toluene and butyl acetate, the reaction temperature is 110.degree. to 120.degree. C., and the ring opening polymerization of .epsilon.-caprolactone to hydroxyl groups is very slow at this temperature. In order to increase the reaction rate, it is necessary to use a large quantity of catalyst for ring opening polymerization. Such a catalyst is usually a tin or titanium compound which discolors considerably the resulting resin. In addition, when the resin is used for painting, the catalyst adversely affects the stability of the paint and the performance of the resulting coating film.
Discoloration is fatal to the acrylic polyol resin which is characterized by the absence of color among many coating resins. Acrylic polyol colored yellowish brown is by no means good for practical use.
In the case where xylene and ethylene glycol monoethyl ether acetate are used as the solvent, it is possible to employ a reaction temperature higher than 140.degree. C. However, if a titanium catalyst, which is highly reactive, is used, an undesirable ester interchange reaction takes place as shown in the following chemical equation. ##STR3##
British Patent No. 1257 638 discloses reaction of 2-hydroxyethyl acrylate with caprolactone in the presence of an acidic catalyst.
In order to synthesize a lactone-modified acrylic polyol which contains only a small quantity of metal catalyst, causes very little discoloration, and permits one to select any solvent, the present inventors carried out a series of intensive studies which led to the development of a production process which is entirely different from the conventional technique. The present invention is based on such development.